Atthemostbasiclevel,asignalamplifierdoesexactlywhatyouexpectitmakesasignalbigger!However,thewayinwhichitisdonevarywiththedesignoftheactualamplifier,thetypeofsignal,andthereasonwhywewanttoenlargethesignal.Wecanillustratethisbyconsideringthecommonexampleofa“Hi-Fi”audiosystem.InatypicalmodernHi-Fi:system,thesignalswillcomefromaunitlikeaCDplayer,FMtuner,oraTape/Minidiskunit.Thesignalstheyproducehavetypicallevelsoftheorderof100mVorsowhenthemusicismoderatelyloud.Thisisareasonablylargevoltage,easytodetectwithsomethinglikeanoscilloscopeoravoltmeter.However,theactualpowerlevelsofthesesignalsarequitemodest.Typically,thesesourcescanonlyprovidecurrentsofafewmilliamps,whichbyP=VImeanspowersofjustafewmilliwatts.AtypicalloudspeakerwillrequirebetweenafewWattsandperhapsover100Wattstoproduceloudsound.HencewewillrequiresomefromofPowerAmplifierto“boost”thesignalpowerlevelfromthesourceandmakeitbigenoughtoplaythemusic.Manypracticalamplifierchaintogetheraseriesofanalogamplifierstagestoobtainahighoverallvoltagegain.Forexample,aPAsystemmightstartwithvoltagesoftheorderof0.1mVfrommicrophones,andboostthistoperhaps10to100Vtodriveloudspeakers.Thisrequiresanoverallvoltagegainof10,soanumberofvoltagegainstageswillberequired.Inmanycaseswewishtoamplifythecurrentsignallevelaswellasthevoltage.Theexamplewecanconsiderhereisthesignalrequiredtodrivetheloudspeakersina“Hi-Fi”system.Thesewilltendtohaveatypicalinputimpedanceoftheorderof8Ohms.Sotodrive,say,100Wattsintosuchaloudspeakerloadwehavetosimultaneouslyprovideavoltageof28Vrmsand3.5Arms.Takingtheexampleofamicrophoneasaninitialsourceagainatypicalsourceimpedancewillbearound100Ohms.Hencethemicrophonewillprovidejust1nAwhenproducing0.1mV.Thismeansthattotakethisanddrive100Wintoaloudspeakertheamplifiersystemmustamplifythesignalcurrentbyafactorofover10atthesametimeasboostingthevoltagebyasimilaramount.Thismeansthattheoverallpowergainrequiredis10i.e.180dB!Thishighoverallpowergainisonereasonitiscommontospreadtheamplifyingfunctionintoseparatelyboxedpreandpoweramplifiers.Thesignallevelsinsidepoweramplifiersaresomuchlargerthantheseweakinputsthateventheslightest“l(fā)eakage”fromtheoutputbacktotheinputmaycauseproblems.Byputtingthehighpower(highcurrent)andlowpowersectionsindifferentboxeswecanhelpprotecttheinputsignalsfromharm.Inpractice,manydeviceswhichrequirehighcurrentsandpowerstendtoworkonthebasisthatitisthesignalvoltagewhichdeterminesthelevelofresponse,andtheythendrawthecurrenttheyneedinordertowork.Forexample,itistheconventionwithloudspeakersthatthevolumeofthesoundshouldbesetbythevoltageappliedtothespeaker.Despitethis,mostloudspeakershaveanefficiency(theeffectivenesswithwhichelectricalpowerisconvertedintoacousticalpower)whichishighlyfrequencydependent.Toalargeextentthisarisesasanaturalconsequenceofthephysicalpropertiesofloudspeakers.Wewontworryaboutthedetailshere,butasaresultaloudspeakersinputimpedanceusuallyvariesinquiteacomplicatedmannerwiththefrequency.(Sometimesalsowiththeinputlevel.)Thiskindofbehaviorisquitecommoninelectronicsystems.Itmeansthat,ininformationtrms,thesignalpatternisdeterminedbythewaythevoltagevarieswithtime,andideallythecurrentrequiredisthendrawn.Althoughtheaboveisbasedonahighpowerexample,asimilarsituationcanarisewhenasenorisabletogenerateavoltageinresponsetoaninputstimulusbutcanonlysupplyaverylimitedcurrent.Inthesesituationswerequireeitheracurrentamplifierorabuffer.Thesedevicesarequitesimilar,andineachcaseweareusingsomeformofgaindeviceandcircuittoincreasethesignalcurrentlevel.However,acurrentamplifieralwaystriestomultiplythecurrentbyasetamount.Henceitissimilarinactiontoavoltageamplifierwhichalwaystriestomultiplythesignalcurrentbyasetamount.Thebufferdiffersfromthecurrentamplifierasitsetsouttoprovidewhatevercurrentlevelisdemandedfromitinordertomaintainthesignalvoltagetoldtoassert.Henceitwillhaveahighercurrentgainwhenconnectedtoamoredemandingload.Thenatureofelectricalsignalsinelectroniccircuitsreadilyenablesthetechnologytobedividedintoclasses.Oneoftheclassesisanalogelectronics；anotherisdigitalelectronics.Bothanaloganddigitalelectronicsusesimilarelectronicelements,butthemannerofuseisdifferent,andthetechnologiesappeartobequitedistinct.Forthisreasonweshallstudythemseparatelyuntilwebringthemtogetherastheyinvariablyuniteininstrumentationandapplications.Analogelectronicspertainstothosesystemsinwhichtheelectricalvoltageandelectricalcurrentareanalogoustophysicalquantitiesandvarycontinuously.Electroniccircuitsthatreproducemusicmusthavevoltagesandcurrentsthatareproportionaltothesound.Ahighfidelityamplifyingsystemattemptstokeeptheanalogyastrueaspossible.Analogelectroniccircuitsarecarefullydesignedtomaketheelectricalvoltagesandcurrentsfollowtheinputsignal.Ifaninputsignaldoublesinamplitude,theoutputvoltageorcurrentalsoshoulddouble；thisispossiblebecausethecircuitelementsaremadetooperatewithinlimitsthatpreservethelinearity.Anelectricalvoltagethatisproportionaltotemperatureandchangessmoothlyasthetemperaturechangesisananalogoftemperature.Ifthetemperaturerangeisdividedintosmallincrements,thenthetemperaturemaybeindicatedbyadigitaldisplay.Asthetemperature(voltage)changessmoothly,adecisionmustbemadebyanelectronicsystemastothenumericalvaluetobedisplayedasthetemperature.Thecircuitmakingthedecisioniscalledananalog-to-digitalconverter,ADC.Theinverseprocessisaccomplishedbyadigital-to-analogconverter,DAC.Digitalelectroniccircuitsdonotrequirethelinearityofanalogcircuits.Digitalcircuitsactaselectronicswitchesandswitchfromonestatetoanother.Theoutputstate,onoroff,istheonlysignalconditiontobeexamined.Indigitalcircuitstheoutputstateisdeterminedbytheinputsignalsinasdirectamannerastheoutputvoltageofananalogcircuitisrelatedtotheinputsignal.Indigitalcircuitstherelationbetweeninputandoutputstatesareexpressedaslogicequations；theelementsofdigitalelectronicsarecalledlogicgates.Logicgatesswitchbetweenstates,onoroff,veryquicklysothattheymayoperateatmanymegahertzincomputersandotherapplications.Astechnicaldevelopmentscontinuetoprovidenewandamazingintegratedcircuits,astheyhavesincethe1960s,bothanaloganddigitalsystemswillbemorecapable.Thedesignersofelectronicsystemsusingintegratedcircuitswillhaveunlimitedpossibilitiesforinnovation.一般情況下，信號放大器正如人們期望的一樣工作-將信號放大。然而，信號的放大方式隨著實際放大器的設(shè)計、信號的類型以及放大信號目的的不同而變化。這一點可以通過一個常見的高保真音響系統(tǒng)實例來加以說明。在典型的現(xiàn)代高保真系統(tǒng)中，信號是來自于CD播放器、調(diào)頻收音機或磁帶/小型磁盤機等設(shè)備。當(dāng)音樂聲大小適當(dāng)時，這些設(shè)備產(chǎn)生信號的幅度大概在100毫伏左右。這種信號幅度相當(dāng)高，易于用示波器或電壓表等儀器檢測到。但是，這些信號的實際能量水平并不高。典型情況下，這些信號源只能提供毫安級的電流。根據(jù)公式P=VI，其功率只有幾十毫瓦。典型的揚聲器需要幾十瓦到數(shù)百瓦的功率才能產(chǎn)生足夠大的聲音。因此，我們需要某些形式的功率放大器來提高來自信號源的信號功率，使其足以播放音樂。許多實際的放大器將多個放大器級聯(lián)起來，以獲得較高的電壓增益。例如，一個功率放大系統(tǒng)的輸入是來自于麥克風(fēng)的0.1毫伏的電壓，將其放大到10伏到100伏才能推動揚聲器。這就要求電壓的總增益達到10的九次方，因此就需要很多放大器級聯(lián)起來。在很多情況下，除了信號的電壓外，我們還要放大信號的電流。這里我們考慮的例子是高保真系統(tǒng)中用來驅(qū)動揚聲器的信號，其典型的輸入電阻約為8歐姆。因此，要驅(qū)動100瓦的揚聲器負載，就要同時提供28伏的電壓和3.5安的電流信號。仍以麥克風(fēng)作為初始信號源為例，典型的源阻抗在100歐左右。因此，麥克風(fēng)在產(chǎn)生0.1毫伏的信號時，提供的電流僅為1納安。這就表示要接受這種輸入信號并去驅(qū)動100瓦的揚聲器，放大電路就必須將信號的電流和電壓同時放大10的九次方倍。這也就意味著總的功率增益為10的18次方，即180分貝。一般都將放大功能分散到單獨設(shè)計的前級放大器和功率放大器中，其原因就在于功率增益很大。功率放大器中的信號幅度比微弱的輸入信號大得多，即使輸出的極微小的泄漏傳輸?shù)捷斎攵耍紩l(fā)一些問題。通過將大功率（大電流）和小功率放大電路分置在不同的單元中，就可以避免輸入信號受到干擾。實際上，許多需要大電流和大功率的設(shè)備往往都在特定的條件下工作，即由信號的電壓決定響應(yīng)的幅度，繼而由設(shè)備吸收其所需要的電流而工作。例如，揚聲器的音量通常是由所加電壓控制。此外，大多數(shù)揚聲器的效率（電能被轉(zhuǎn)換為聲能的效能）基本上與頻率無關(guān)。在很大程度上，這是由揚聲器的物理特性所產(chǎn)生的自然結(jié)果。這里不必考慮具體的細節(jié)，但揚聲器的輸入阻抗隨頻率的不同而呈復(fù)雜的變化（有時也與輸入信號的幅度有關(guān)）。這種特點在電子系統(tǒng)中很常見。用信息術(shù)語來說就是信號類型取決于電壓隨時間變化的情況，且在理想情況下就能吸收所需的電流。盡管上述情況是基于大功率的例子，但當(dāng)傳感器在輸入的激勵下做出響應(yīng)而產(chǎn)生一定的電壓，卻只能提供有限的電流時，類似的情況也會出現(xiàn)。這時我們就需要一個電流放大器或緩沖器。這些裝置非常相似，在各種情況下都可采用一定形式的增益裝置或電路來提高信號電流的大小。不過，電流放大器總是設(shè)法對電流進行一定的放大。這與電壓放大器的功能相似。緩沖器總是可以提供任何你所需要的電流，以便維持其